This invention relates generally to the testing of printed circuit boards and, more particularly, to board test fixtures and other mechanical interfaces for electrically interconnecting electronic circuit boards and the like to electrical test systems.
A board test system consists of numerous electronic drivers (sources) and receivers (detectors) which are connected through an electronic switching mechanism, or scanner, to a plurality of contact points referred to as scanner pins. A board test fixture then provides an interface between these scanner pins and the electronic components located on an electronic circuit board. Because the electronic test signals which are used to determine whether the electronic component is operating properly must pass through the board test fixture both on their way to and from the electronic component, the board test fixture must maintain the signal quality of the test signals to ensure that the electronic component is not incorrectly diagnosed as operating either properly or improperly. It has been recognized in the art that in order to ensure maximum test signal quality, the length of the signal path between the scanner and the electronic circuit board must be kept as short as possible. The factory normally dictates a short vertical coupling of the test system and the circuit board. In other words, a xe2x80x9cshort-wirexe2x80x9d board test fixture is designed to sit directly on top of the scanner and the electronic circuit board directly on the board test fixture. Furthermore, any board test fixture must be easy to assemble and maintain in order to be readily usable and cost effective. Finally, the ability to automate the assembly of the fixture is also an important feature. Prior art short-wire length test fixtures such as U.S. Pat. No. 5,304,921, issued to Cook et al, have a common configuration. Referring to the Cook prior art of FIG. 1, a circuit board 202 to be tested is placed on top of a board test fixture 204 via the guidance of an alignment pin 205. The board test fixture 204 provides an electrical interface between the circuit board 202 and the board test system 206. In other words, many test signals travel through the board test fixture 204. The Cook invention implements a grounding system which utilizes ground coax assemblies 208 and a plated alignment plate 210 in the board test fixture 204, as are described in detail below.
Various test signal paths travel from the board test system 206 through the board test fixture 204 to electronic components 212, 214 on a circuit board 202 under test. As shown in FIG. 1, for example, a test signal from the board test system 206 travels through the wire wrapping pin 216 of a personality pin 218, through a wire 220, and then through a signal probe 222, respectively, before reaching the electronic component 212. The location of the personality pins and the signal probes as well as the length of their connecting wires changes depending upon the orientation of electronic components 212, 214 on the circuit board 202 under test.
Various ground paths also travel through the prior art board test fixture 204 between the circuit board 202 under test and the board test system 206. Ground connectors from the board test system 206 contact the bottom of the board test fixture 204 in order to make the ground path available to the fixture 204. At the bottom of the fixture 204 is the alignment plate 210, which serves as a focus means to capture and guide personality pins 218, which carry test signals, into an organized orthogonal array-like pattern when the alignment plate 210 is installed so that the board test system 206 can send test signals through the fixture 204.
The Cook patent provides an enhanced grounding system for a short-wire length test system. The enhanced system utilizes an internal ground plane 230 located in the test fixture and interposed between the probe plate 242 and alignment plate 210. The internal ground plane 230 is problematic in that it becomes difficult to align the ground plane during assembly of the test fixture. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected, and the corresponding array of test probes is configured in the test fixture. This typically involves drilling a pattern of holes in a probe plate to match the customized array of test probes and personality pins and then mounting the test probes and personality pins in the drilled holes on the probe plate. In Cook, a separated ground plane is provided, which requires a separate drilling step to align the personality pins and signal probes through the ground plane and corresponding probe plate. It is a difficult task to achieve the required perfect alignment and becomes even more difficult when dealing with high node count circuit board assemblies. Furthermore, Cook utilizes a detached ground plane which increases signal path lengths, adds weight and significantly reduces the overall space availability within the cavity of the board test fixture. An increased signal path length between the scanner and electronic circuit board also introduces both crosstalk and signal loss between connection points.
The present invention is an enhanced probe plate assembly contained in a standard circuit board test fixture. The probe plate assembly comprises conductive surface(s) laminated to the underside of a probe plate which are used for grounding, voltage sources, and common interconnection grids. By providing an integrated conductive surface or surfaces a separate drilling step and prior art alignment problems are eliminated. Holes drilled in a probe plate for personality pins and probe pins also result in perfectly aligned holes in the conductive surface, since the conductive surface is now directly attached to the probe plate.
Furthermore, the shortening of the signal path between the scanner and electronic circuit will ensure maximum signal quality, reduce weight, and significantly increase the overall space available within the cavity of the tester. Also, the ability to place conductive surfaces conveniently on the probe plate results in reduced length probe to personality pin hard wire connections and the flexibility of providing ground planes, voltage sources, and common interconnection grids anywhere on the probe plate. In fact, grounding planes, common interconnection grids, and voltage sources can coexist on one probe plate. This is accomplished by providing sections of conductive surface(s) throughout the underside of the probe plate.